|Publication number||US7538860 B2|
|Application number||US 11/840,363|
|Publication date||May 26, 2009|
|Filing date||Aug 17, 2007|
|Priority date||Aug 17, 2007|
|Also published as||US20090046276|
|Publication number||11840363, 840363, US 7538860 B2, US 7538860B2, US-B2-7538860, US7538860 B2, US7538860B2|
|Inventors||Jason P. Moore|
|Original Assignee||The United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (27), Referenced by (5), Classifications (8), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention generally relates to the use of in-fiber Bragg gratings sensors and, more particularly, relates to systems and methods for determining a reflection wavelength of multiple low-reflectivity Bragg gratings in a sensing optical fiber.
A Bragg grating is a periodic variation in the refractive index in a small section of optical fiber cable. As light propagates along the fiber, a very narrow range of wavelengths is reflected by the Bragg grating while all other wavelengths are transmitted through the grating. The center of this reflected band is called the Bragg wavelength. In-fiber Bragg gratings sensors have been utilized as measuring devices for strain, temperature, pressure, and chemical presence, as examples. Typically, the measurements are based on identifying the peak reflection wavelength of a grating or interrogating the reflection or transmission wavelength spectrum of a grating, and then inferring strain effects based on the shifting of each grating's peak reflection wavelengths or spectra. As strain sensors, the fiber containing Bragg gratings sensors is typically bonded to the surface of a test article or is embedded within the material under test. As the test article is subjected to loading or other strain-inducing phenomena, the fiber experiences an induced strain. The induced strain at a Bragg grating sensor area causes the peak reflection wavelength of that sensor to shift, generally in a linear fashion, in relation to strain. Bragg grating sensor systems typically perform temperature measurements by inferring temperature changes after measuring fiber strain that occurs due to the fiber's coefficient of thermal expansion. Pressure and chemical sensor systems perform similarly, as they typically measure in-fiber Bragg grating sensor strain which is induced through the physical or chemical changes of materials around the sensing fiber. It is to this end that systems have been developed for the purpose of measuring or calculating the peak reflection wavelengths of in-fiber Bragg grating sensors. Because the information from a Bragg grating sensor can directly relate hazardous structural or environmental conditions to the appropriate safety interests and because properties under test can exhibit rapid changes, it is desirable to perform these wavelength measurements as quickly and as numerously as possible while maintaining acceptable accuracy.
Known methods and systems for determination of the spectra and/or the peak reflection wavelengths of multiple Bragg grating sensors in a single fiber use Optical Frequency Domain Reflectometer (OFDR) technology. Current OFDR systems derive wavelength information of Bragg gratings in a single fiber by sampling and storing the reflections from a sensing fiber while sweeping the wavelength of a system source laser. The recorded data is then manipulated through processor-intensive algorithms, such as Discrete Fourier Transforms, data filtering/smoothing, and threshold/peak detection. Such processing adds significant delay in the determination of Bragg grating reflection wavelengths, which in turn delays the measurement data repetition rates.
One known OFDR system and algorithm is capable of interrogating multiple (hundreds) Bragg grating sensors on a single fiber with the nominal wavelengths of the sensors being equal. Prior to the use of OFDR, Bragg grating sensors were interrogated using wavelength division multiplexing (WDM) systems which required the nominal reflection wavelengths of the sensors to be non-equal. WDM systems were also limited in the number of sensors they could interrogate on a single fiber. The OFDR technology allows for the interrogation of several hundred Bragg grating sensors all located on one fiber and all nominally having equal reflection wavelengths. The extensive processing of a large data set and the required analog-to-digital (A/D) conversion rates are the main limitations in the speed of the measurement using OFDR technology
Techniques which are less processor-intensive than the current OFDR method exist; however, they are limited in the number of Bragg grating sensors they are able to interrogate from one fiber. Selecting an appropriate Bragg grating sensor system usually involves assigning priority between interrogation speed, sensor number, and measurement accuracy. A system with high data repetition rates as highest priority will usually have a lower sensor quantity capability, reduced accuracy, or both. A system such as the OFDR technology, which is capable of measuring several sensors on one fiber, has high accuracy, but low data repetition rates. The most desirable system is one that combines high speed repetition rates with multiple sensor capability and accuracy comparable with other state-of-the art systems.
One method of interrogating in-fiber Bragg grating sensors using traditional Optical Frequency Domain Reflectometer (OFDR) technology is disclosed in U.S. Pat. No. 6,566,648, issued May 20, 2003, to Froggatt, the contents of which are incorporated herein in their entirety. The method disclosed in Froggatt is capable of measuring many numbers (hundreds) of Bragg grating sensors, but has undesirable processor-intensive algorithms which limit its speed capabilities. Froggatt's OFDR method utilizes a monotonically wavelength sweeping, continuous output, high coherence laser as the source for the sensing fiber and a separate fiber network which contains the necessary fiber optic components to generate calibrated interference fringes which are used to clock the sampling of the sensing fiber reflections through an A/D.
Providing the desired capability (high speed, high sensor number, and high accuracy) has been accomplished by stacking, or paralleling, multiple systems. A WDM-based system, for example, can read several Bragg grating sensors by having multiple source lasers to provide wide bandwidths, or., as is usually instituted, multiple fibers are connected to one system. An OFDR system can provide pseudo-high-speed capability and still interrogate several hundred sensors on one fiber by storing data and deriving measurements in a post-processing fashion. The latter method still does not provide useful measurements in real-time, so the high speed capability is still not completely realized. An effort to improve the processing speed of current OFDR technology through improved processing hardware and software algorithms is underway and has resulted in increased measure speeds; however, the inherent requirement of an OFDR system to sample data and perform Discrete Fourier Transforms and other related algorithms restricts the technology to be limited by A/D conversion speeds and processor speeds.
The object of the present invention is to overcome the aforementioned drawbacks of OFRD-based Bragg grating sensor systems and to provide high interrogation speed, sensor number, and measurement accuracy. The system and method of embodiments of the invention yield a system capable of determining the spectra and/or peak reflection wavelengths of multiple Bragg gratings on a single sensing optical fiber without the use of processor-intensive, post-sampling algorithms. Measurements and determinations are performed in real-time, as the OFDR source laser is swept through its wavelength range, resulting in significantly higher data repetition rates.
In one embodiment of the invention, a system is provided for determining a reflection wavelength of multiple low-reflectivity Bragg gratings in a sensing optical fiber having a broadband reflector and a plurality of the Bragg gratings at different distances from the broadband reflector along a length of the sensing optical fiber. The system comprises: (1) a source laser coupled to the sensing optical fiber; (2) an optical detector coupled to the sensing optical fiber and configured to detect a reflected signal from the sensing optical fiber; (3) a plurality of frequency generators, each frequency generator configured to generate a signal having a frequency corresponding to an interferometer frequency of a different one of the plurality of Bragg gratings; (4) a plurality of demodulation elements, each demodulation element coupled to the optical detector and to a different one of the plurality of frequency generators, each demodulation element configured to combine the signal produced by a different one of the plurality of frequency Generators with the detected signal from the sensing optical fiber; (5) a plurality of peak detectors, each peak detector coupled to a different one of the demodulation elements and configured to detect a peak of the combined signal from a different one of the demodulation elements; and (6) a laser wavenumber detection element coupled to the peak detectors and configured to determine a wavenumber of the laser when any of the peak detectors detects a peak.
In one embodiment of the invention, the plurality of frequency generators each comprise a frequency synthesizer. In an alternative embodiment, the plurality of frequency generators each comprise an optical fiber interferometer, each interferometer comprising two lengths of optical fiber having a difference in length substantially equal to the distance between the broadband reflector and a different one of the plurality of Bragg gratings.
The laser wavenumber detection element may comprise: (1) a reference optical fiber network coupled to the source laser and configured to provide a wavenumber counting signal; (2) a calibrated optical fiber network configured to reflect light from the source laser when the source laser is emitting light at a predetermined frequency; (3) a counter coupled to the reference optical fiber network and the calibrated optical fiber network, the counter being clocked by the wavenumber counting signal and triggered by the reflection of light from the calibrated optical fiber network; and (4) a plurality of registers each register configured to capture and store an output value from the counter when a different one of the peak detectors detects a peak.
The source laser may comprise a high coherence, monotonically wavelength sweeping, continuous output, mode-hop free, fiber-coupled laser.
In one embodiment of the invention, each demodulation element comprises an analog multiplier and a low-pass filter. In an alternative embodiment, each demodulation element comprises an analog mixer and a low-pass filter.
In addition to the system for determining a reflection wavelength of multiple low-reflectivity Bragg gratings in a sensing optical fiber having a broadband reflector and a plurality of the Bragg gratings at different distances from the broadband reflector along a length of the sensing optical fiber as described above, other aspects of the present invention are directed to corresponding methods for determining a reflection wavelength of multiple low-reflectivity Bragg gratings in such a sensing optical fiber.
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
As in traditional OFDR systems and methods, embodiments of the present invention are capable of determining the spectra and/or peak reflection wavelengths of multiple Bragg gratings on a single sensing optical fiber. The sensing fiber comprises an inline partial broad band reflector and any number of Bragg gratings distributed along the fiber. When the source laser is swept in wavelength, this sensing fiber arrangement causes the reflection amplitude spectra of each Bragg grating sensor to amplitude modulate an interferometer beat frequency corresponding to the distance of each Bragg grating sensor from the broadband reflector along the length of the sensing optical fiber. Unlike traditional OFDR systems and methods, the system and method of embodiments of the present invention make use of a plurality of frequency generators. One frequency generator is provided for each Bragg grating, and each frequency generator produces a signal having a frequency corresponding to an interferometer frequency of a different one of the plurality of Bragg gratings. In one embodiment of the invention, the frequency generators are provided by an additional fiber optic networks termed an “oscillator” fiber network (each separate section of the network is termed an “oscillator”), that is coupled to the source laser. The oscillator network comprises a plurality of individual fiber optic interferometers (described in more detail below), one for each Bragg grating sensor and each matching the interferometer length between one of the Bragg grating sensors and the broadband reflector in the sensing fiber. The return from the sensing fiber and the returns from each individual oscillator interferometer are detected by individual photo-detectors and amplified. When the laser is frequency swept, the interferometer beat frequencies of the Bragg grating sensors each has a matching oscillator interferometer beat frequency. The detected output of the oscillator interferometer amplifiers are coupled with the detected output of the sensing fiber amplifier through electronic analog demodulation circuitry. Any known demodulation technique may be used. In one exemplary embodiment, the demodulation circuitry comprises an analog multiplier or mixer and a low pass filter. Because the oscillator interferometers match the lengths of individual Bragg grating sensor length interferometers, the spectrum of each sensor is multiplied, or mixed, down into the DC frequency range in the time domain at the output of each multiplier. A low pass filter module on the output of each multiplier module filters out other Bragg grating spectra and other information not pertaining to the particular Bragg grating sensor that is associated with each demodulator's input oscillator frequency. In this arrangement, each demodulation segment outputs the wavelength reflection spectrum of a Bragg grating sensor as the laser is wavelength swept. To determine reflection wavelength, the output spectra of each demodulator are subjected to peak detection circuitry, such as simple threshold detection or otherwise. A laser wavenumber detection element is configured to determine a wavenumber of the laser when peaks are detected. The laser wavenumber detection element may comprise a reference interferometer, a digital counting circuit, and a plurality of registers. The reference interferometer, which is used in the traditional OFDR technology, is used to provide a wavenumber “counting” signal. Because the reference interferometer length is calibrated, the wavenumber period of the fringes of the reference interferometer are known, which enables the reference signal to drive a digital counting circuit whose digital output can be stored in registers when triggered by the peak detection circuitry. The counter is disabled until a pulse from a “trigger” network (the wavelength reference grating fiber described below) enables counting. The trigger network contains a high reflectivity fiber Bragg grating whose reflected and transmitted optical power is detected, amplified, and compared to produce a digital “low” when not reflecting and a digital “high” when reflecting. In this configuration, the counter is enabled at a reliable, repeatable wavenumber, and the counter module provides a digital encoding of the wavenumber as the laser wavelength is swept. At the end of each sweep of the laser, each register contains the digitally encoded reflection wavenumber of each grating. To determine strain, minimal logistical hardware is needed to compare measured wavenumber values (i.e., when a peak is detected) to zero-strain, baseline wavenumber values.
Referring now to
The 95% of the optical power that is directed to the sensing section 28 is first directed through a one-by-two 50/50 coupler 26. Coupler 26 directs optical power to the sensing fiber 16 and couples the reflected optical power from the sensing fiber back through the coupler and to a photo-detector 38. The signal from the photo-detector 38 provides a “sensor return” signal to the analog electronics (as discussed in conjunction with
Optical power is directed to each of a plurality of “oscillator” interferometers 30 through a 50/50 coupler 48 (for simplification purposes, the components of only one of the oscillator interferometers are labeled). The 50/50 coupler 48 directs optical power through two fibers 50, 52 of different lengths, both terminated with a Faraday rotator mirror (FRM1A and FRM1B respectively), and couple the reflected optical power from the two lengths back through the coupler and onto a photo-detector 54. The signal from the photo-detector 54 provides an “oscillator” signal to the analog electronics (as discussed in conjunction with
The wavelength reference grating fiber 34 comprises a high reflectivity grating with a calibrated reflection wavelength. It is fusion spliced to a coupler 56 in such a way that reflected and transmitted optical power are detected at photo detectors 58, 60, respectively, and amplified for comparison. As discussed in more detail below, the wavelength reference (rating fiber provides the digital pulse to start the digital counter and the clocking of the A/D converters for spectral sampling. This serves as a calibrated reference for each laser scan.
The interrogation of the sensing fiber begins with the laser emitting at the lower wavelength of the laser's wavelength scanning range. The laser then scans upward in wavelength and stops at the high wavelength of the wavelength scanning range. The laser has a monotonic sweeping characteristic (i.e., as the wavelength is being swept, there is substantially no jitter or backlash or any other type of behavior that might cause a negative sweeping derivative during scanning). The laser is also a continuous output laser in which there is substantially no dropout of power during the wavelength sweep. The optical output of the laser is coupled directly to the optical network, as described above. As the laser sweeps wavelength, the gratings reflect a particular wavelength of optical power according to their own individual strain conditions. When a grating is reflecting optical power, an interferometer condition is established between the orating and the broadband reflector. The reflection spectrum will amplitude modulate the sinusoidal interference output of the broadband reflector-grating pair. In this way, the reflection spectrum of each grating is returned through the coupler as an amplitude-modulated sinusoidal optical signal whose frequency is dependant on the length of fiber between the broadband reflector and each grating. As the laser is wavelength-swept, each oscillator interferometer creates an optical “oscillator” signal having an interferometer frequency closely matching the interferometer frequency of one of the gratings in the sensing fiber.
The reflected power from the sensing fiber, the reflected power from the oscillator interferometers, the reflected power from the reference interferometer, and the transmitted and reflected optical power from the wavelength reference grating are all connected to the analog electronics module 18 for detection and amplification, as illustrated in
The outputs of the demodulation circuits (“grating 1 spectrum,” “grating 2 spectrum,” “grating 3 spectrum,” . . . “grating N spectrum”) may also be connected to an A/D module 22 for sampling/storage in the fashion previously outlined by Froggatt, if so desired. It is noted, however, that the purpose of the present invention is to yield measurement data without the need for sampling signals and performing digital processing. The sampling methodology of Froggatt is included here to illustrate that embodiments of the present invention do not eliminate the capability to perform Froggatt's sampling technique. The start_sample signal and the reference_clock signal may be connected to the A/D & Storage module 22 in order to perform this task. As discussed in Froggatt, the A/D conversion is initiated by the start_sample signal transitioning to digital “high.” The start_sample signal is digital “high” when the laser sweeps through the predetermined reflection wavelength of the wavelength reference grating fiber 34 (i.e., when the laser is emitting at the same wavelength as the predetermined reflection wavelength of the reference grating fiber). The wavelength reference grating fiber 34 is carefully calibrated (e.g., with National Institute of Standards and Technology traceability) such that the reflection wavelength is known and repeatable. This enables the initiation of the A/D conversion to be repeatable at the same laser wavelength over multiple scans of the laser. The reference_clock signal has a period directly related to the length of the reference interferometer 32. The period is represented as 1/(2nLref), where n is the index of refraction of the fiber making up the reference interferometer and Lref is the length difference, in meters, of the reference interferometer. The units of the phase are meters−1. This relationship maps the basis of the sampled set into the wavenumber domain, with the basis zero point being 1/(reference grating trigger wavelength), and each sample being 1/(2nLref) apart. Sampling each demodulation circuit output in this way yields the relative optical reflection amplitude versus wavenumber for each grating in the sensing fiber.
As mentioned above, the outputs of the demodulation circuits (“grating 1 spectrum,” “grating 2 spectrum,” “grating 3 spectrum,” . . . “grating N spectrum”) are connected to the peak detection electronics module 20. Each output spectrum is either peak-detected or edge-detected as the source laser is wavelength-swept in order to produce a digital signal that is “high” when the peak is detected (these peak detection signals are labeled in
As illustrated in
Other inputs, outputs, and components not shown may include, but are not limited to, signals to indicate the scanning direction of the laser (increasing or decreasing wavelength), connections for extracting the digital information stored in the registers, hardware to eliminate timing, conflicts between the digital counter and the registers, and hardware for comparing measured values to baseline values to yield absolute results.
The source laser may be any laser whose characteristics are: (1) monotonic in wavelength tuning (up or down); (2) continuous output; (3) fiber coupled or capable of being fiber coupled; (4) mode-hop free during tuning; and (5) highly coherent (i.e., enough coherence to provide adequate interference for the longest interferometer in the optical network). Some lasers have a self-scanning control capability in which the laser will scan up and down in a wavelength range entered by the user, while others require a control input to initiate a wavelength sweep. The latter typically requires that the user or a control processor program the laser as to the wavelength at which this line will transition to “high”. Some lasers have a digital wavelength output line which can be used in place of the start_sample digital line used to clear the digital counter and grating registers at the start of a measurement. The connections from optical module to optical module can be accomplished through fusion splicing, connectors, or any other means in which back reflections do not cause significant signal errors. The sensing fiber can have any number of gratings at any distance from the broadband reflector so long as sufficient interferometer behavior exists. The distance between gratings on the sensing fiber can be as small as needed so long as the low-pass filter circuitry in the detection and analog electronics module can still sufficiently suppress unwanted “other” grating spectra. The sensing fiber does not have to use Bragg gratings. The invention described here can be used to determine the spectrum of any reflective artifact or otherwise in optical fiber. The interferometric signal formations in the optical network can be accomplished a variety of ways, including, but not limited to, using polarization-maintaining fiber in place of circular-core fiber, using a two-coupler configuration, or on a substrate as would be done for miniaturization of the optical network. The sensing fiber can consists of multi-core fiber. Polarization-maintaining fiber may be used anywhere in the fiber optic network or sensing fiber. The optical power detection and electronic amplification methods are not specific to this invention and may be accomplished using any known means. The mixing/multiplying circuitry and low-pass filter methods are not specific to this invention and may be accomplished using any known means. The digital counting and register configurations are not specific to this invention and may be accomplished using any known means. Although not illustrated, power supply and associated noise-reduction hardware are generally required in any electronics application. Also not illustrated is hardware for extracting the stored counter values from the grating registers or associated processing hardware to convert these digitally stored values into a wavenumber measurement.
Referring now to
The fiber optic network of this alternative embodiment is illustrated in
To further illustrate the improvements of the present invention over the prior art,
In the prior art of
Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
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|U.S. Classification||356/35.5, 356/478|
|Cooperative Classification||G02B6/2932, G01N21/7703, G02B6/02076|
|European Classification||G02B6/02G8, G02B6/293D4F2C|
|Aug 17, 2007||AS||Assignment|
Owner name: UNITED STATES OF AMERICA AS REPRESENTED BY THE ADM
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:MOORE, JASON P.;REEL/FRAME:019710/0001
Effective date: 20070817
|Sep 21, 2012||FPAY||Fee payment|
Year of fee payment: 4